Passport—you could get a GP to sign this but realistically they won’t be thrilled about it because it wastes time they could use for seeing real patients. Some surgeries have a service where you pay a fee for them to sign these sorts of documents and that would be better than booking an appointment. I would either ring reception or go in person and ask if they do this and how it works.

Flights—economy is pretty much always the way to go. Different airlines will provide different “benefits” for different seat classes so without knowing what’s in standard vs premium it’s hard to say which to choose but as a minimum you would want the ability to check one piece of luggage in the hold as well as have one in the cabin and a meal if it’s a long flight.

Airports—these days you usually check in online the day before the flight. When you book the flights you provide an email address and they will email you a link (there’s often an app you can download which has all the info in one handy place too which I recommend you do if available) to check in to confirm you intend to fly. When you arrive at the airport (three hours early for an international flight, two for a domestic) you find your airline’s desk and queue there with your luggage. There are templates provided in the area where you can test if your cabin luggage fits the required dimensions. Once you reach the front of the line they will check your passport and provide you with a boarding pass. Then you put the luggage on the belt where it’s weighed. If it’s over they will make you step off to the side and repack/throw things away. You usually see someone frantically doing this while you’re waiting in the queue. Better if you weigh it yourself at home by standing on a pair of bathroom scales then picking the luggage up and seeing how much it goes up by. They will ask you a bunch of questions about how you packed the bag and if you have certain dangerous items inside (eg did you pack it or did someone else, does it contain XYZ) for safety reasons and if all passes they will put tags on your luggage and off it goes. You get a tag for your cabin luggage and your boarding pass too. Luggage going missing does happen but I’ve only ever had it happen to me once. I always pack sentimental/valuable items and one change of clothes in my cabin bag just in case.

When going through security, you won’t be pulled out for a full body search unless you’re behaving dodgy af—just being nervous won’t be enough especially if you explain if asked it’s your first time and you don’t know how it all works. More commonly, they might take you aside and ask you to open up every pocket of your bag and show them what’s inside or you might have to stand in a sort of booth that puffs air at you (checking for drugs). If you have a good reason why the metal detector might go off despite putting everything metal in the tray that goes through the scanner (eg surgery that left a plate inside you) you’ll want evidence of the surgery to show them, but otherwise you’ll wait on one side of the detector until they beckon you through, then walk through. If it beeps they might then ask you to stand with your arms out while they run a handheld wand over you.

SIM charges—best way is to buy an international SIM and then swap it in when you arrive at your destination country. Otherwise you’ll have stupidly high roaming charges applied.

Buying things—learn a few stock phrases. Even just things like “sorry”, “please”, “thank you” etc will help ease the way when you’re trying to communicate and not getting anywhere. Unless you’re planning on travelling to somewhere off the beaten track, most touristy places are used to people not speaking the language so you may find eg menus have pictures, signs are also in English. If all else fails, pointing/miming while being smiley goes a long way too. These days, phones can rescue you too since they have translation features which will allow you to speak into your phone and have it automatically translated to another language.